Mechanical properties of a co-extruded Metallic Glass/Alloy (MeGA) rod—Effect of the metallic glass volume fraction

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Abstract

A Metallic Glass/Alloy (MeGA) rod with a core in zirconium-based bulk metallic glass and a sleeve in aluminium alloy has been successfully elaborated by co-extrusion. SEM observations of the cross-section of the rod show that the interface between the glass and the alloy is defect-free. Compression tests are carried out at room temperature on the MeGA rods containing various glass volume fractions. The yield stress is well described by the rule of mixtures which combines the strength of the glass and that of the alloy, suggesting isostrain behaviour as could be expected. During compression, a good mechanical bonding is observed in the MeGA-rod even after the first fracture of the metallic glass. Finally, push-out tests are performed to evaluate the bonding quality between the two materials. Large values of the shear strength are measured which confirms that co-extrusion leads to good bonding between the glass and the aluminium alloy.

Introduction

Bulk metallic glasses (BMGs) are promising materials owing to their spectacular mechanical properties such as a large elastic domain and high fracture stresses [1]. With such properties, BMGs are also interesting candidates as reinforcements in conventional alloys in order to take advantage of the high strength of the glass and of the large ductility of the alloy in a similar way as in ceramic fibre reinforced alloys. In the past, ceramic fibre reinforced materials have been extensively studied thanks to the optimisation of various elaboration processes like liquid pressure infiltration [2] or diffusion bonding [3]. However, these processes require high temperatures in order for the reinforcement to be well bonded with the matrix. An alternative process is composite extrusion in which the composite is formed inside a special die by feeding the continuous fibres into the metal flow during the extrusion process [4]. This process has been experimented with ceramic fibres [5] and metallic wires [6]. In the case of two metallic alloys, co-extrusion is also a process which has been developed quite extensively to manufacture bimetallic rods and tubes [7]. Bimetallic co-extrusion has been used also to manufacture commercial superconductors [8]. This technique is however difficult to perform when a brittle material (like a ceramic) is used for the core of the co-extruded rod. One advantage of a BMG in this framework is its ability to deform intensively under low stresses in the supercooled liquid region (SLR). Depending on the BMG, this region can correspond roughly to the conventional extrusion temperatures of light alloys.

A preliminary work has recently shown that Metallic Glass/Alloy (MeGA) rods with a core in BMG (zirconium and magnesium based BMGs) and a sleeve in conventional light alloy (aluminium and magnesium alloys) can be elaborated by co-extrusion carried out at temperatures corresponding to the supercooled liquid region of the BMG [9]. First results support the idea that interesting mechanical properties can be achieved in such composite materials since defect-free interfaces can apparently be generated between the glass and the light alloy as observed by SEM. The aim of this paper is to investigate further the mechanical properties of such co-extruded metallic glass/alloy (MeGA) rods in the case of a zirconium-based BMG used as the core and an aluminium alloy used as the sleeve with a particular attention given to the effect of the glass volume fraction on the mechanical properties at room temperature.

Section snippets

Experiments

The Zr-based BMG (Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 (at.%), so-called Vit1) was supplied by Howmet Corp. (USA) in the form of a sheet of 3 mm thickness. The amorphous nature of the alloy was confirmed by X-ray diffraction (XRD) and differential scanning calorimetry (DSC) carried out with a heating rate of 10 K/min. The glass transition temperature Tg, the temperature of the first crystallization peak Tx and the SLR defined by ΔT (=Tx  Tg), were measured and values equal to 628, 708 and 80 K were

Results

Typical SEM observations of the cross-section of the co-extruded MeGA rod are shown in Fig. 1a and b. Fig. 1a shows that the glass core is well centered in the rod. At this scale, the interface quality appears quite satisfactory. Fig. 1b shows an observation of the glass/alloy interface at a higher magnification: the quality of the interface appears still fairly good, suggesting that a satisfactory bonding between the glass and the alloy has resulted from extrusion.

Fig. 2 displays the variation

Conclusion

Metallic [glass/alloy] (MeGA) rods with a core in zirconium-based BMG and a sleeve in Al alloy were elaborated by co-extrusion carried out at a temperature corresponding to the supercooled liquid region of the glass. By this process, good interfaces were obtained between the glass and the Al alloy. Compression tests carried out at room temperature have shown that the mixture rule involving the fracture stress of the glass and the compressive strength of the sleeve alloy can describe the

Acknowledgement

The authors thank Gilles BOUTET from SIMAP laboratory for his technical assistance in the co-extrusion tests.

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